1. Technical Field
The present invention relates to a sintered ferrite material to be used for a core material of wire wound components including an inductor to be used for a power circuit or the like, an antenna such as a bar antenna, a transformer and the like, and in particular to a sintered ferrite material, which satisfies high initial permeability, high saturation magnetic flux density and high resistivity, a wire wound component, and a producing method of the sintered ferrite material.
2. Description of Related Art
Conventionally, a Ni-based ferrite material having high resistivity has been used for a ferrite material, which is to be used as a core material of wire wound components including an inductor to be used for a power circuit or the like of a DC-DC converter, an antenna such as a bar antenna, a transformer and the like, in order to ensure electrical insulation from a conductive material.
However, a Ni-based ferrite material, which contains expensive Ni as a main component and has a large magnetostriction constant, has a problem that the soft magnetic property is changed by stress on the core in the process of resin cure in a wire wound component of a resin mold type.
A Li-based ferrite material is known as a material, which does not contain expensive Ni. A Li-based ferrite material having a small magnetostriction constant is characterized in that the rate of change of the soft magnetic characteristic is small even when the Li-based ferrite material is used for a wire wound component of a resin mold type or the like.
However, when a Li-based ferrite material is sintered at a high temperature equal to or higher than 1000° C., it is difficult to obtain high resistivity (equal to or higher than 106 Ωm, for example), though high initial permeability (equal to or higher than 200, for example) and high saturation magnetic flux density suitable for the use in an inductor, an antenna or the like can be obtained.
For example, a Li-based ferrite material for a non-reciprocal circuit element, which has a composition formula of (1-x)Li2O.(5-2α-x)Fe2O3.4xZnO.4yMnO.4βBi2O3, satisfies 0≦α≦0.35, 0≦x≦0.45, 0≦y≦0.2 and 0≦β≦0.005 and has composition wherein at least two of x, y and β are not zero simultaneously and α is not zero when y is zero, has been proposed as a Li-based ferrite material to be sintered at a high temperature equal to or higher than 1000° C. (Japanese Examined Patent Application Publication No. S55-27015).
It is stated in Japanese Examined Patent Application Publication No. S55-27015 that a Li-based ferrite material therein has characteristics, such as a temperature characteristic and a loss characteristic, enhanced by the condition that at least two of Zn, Mn and Bi are contained simultaneously and further has resistivity enhanced by the condition that heat treatment is applied at least one time in nitrogen and is applied for the last time in oxygen in the process of repeated heat treatment in oxygen and nitrogen.
However, a Li-based ferrite material according to Japanese Examined Patent Application Publication No. S55-27015 has low resistivity in the order of 107 Ωcm (105 Ωm), though saturation magnetic flux density equal to or higher than 4000 G (400 mT) can be obtained. Moreover, heat treatment needs to be applied repeatedly in oxygen and nitrogen in order to enhance the resistivity, causing a problem that the manufacturing cycle increases in length and the manufacturing cost increases.
On the other hand, a Li-based ferrite material, which has resistivity enhanced to 106 Ωm or higher by adding a sintering agent such as Bi2O3 and sintering the material at a low temperature (approximately 900° C.), has been proposed (Japanese Patent Application Laid-Open No. 2004-153197). However, such a Li-based ferrite material has a problem that the initial permeability is low.
Moreover, the use of a Li-based ferrite material, which is characterized by having a high squareness ratio, for a memory core material or the like has been conventionally considered. However, a Li-based ferrite material has a problem that the saturation magnetic flux density is lower than that of a Ni-based ferrite material. Accordingly, the saturation magnetic flux density needs to be enhanced for the use in an inductor, an antenna or the like, which requires a satisfactory DC superposition characteristic.